1. Field of the Invention
The invention related to processes for the hydrohalogenation of myrcene.
2. Brief Description of the Prior Art
The literature is replete with descriptions of processes for the hydrohalogenation of conjugated dienes. Representative of such descriptions are those found in the U.S. Pat. Nos. 2,882,323 and 3,016,408 and in British Pat. No. 896,262.
The present invention is particularly advantageous when used to hydrochlorinate myrcene. Myrcene is a conjugated diene of the formula: ##STR1## When hydrochlorinated in the absence of any catalyst, the major product is myrcenyl chloride. However, the commercially valuable products of myrcene hydrochlorination are the associated co-products, namely, geranyl chloride and neryl chloride. Hydrochlorination in the presence of a copper-containing catalyst shifts the reaction in faor of the desired co-products. It has beenpostulated that the hydrochlorination of myrcene in the presence of a copper catalyst proceeds according to the reaction scheme: ##STR2## Cyclization may also undesirably occur to form the alpha-terpinyl chlorides.
When the copper catalyst employed is in the form of cupric chloride (CuCl.sub.2) the products generally include substantial proportions of linalyl chloride and lesser proportions of the desired geranyl and neryl chlorides. When the copper catalyst is in the form of cuprous chloride, the linalyl chloride product is lessened due, apparently, to partial isomerization to the desired geranyl,and neryl chlorides.
From the above proposed reaction scheme, it will be appreciated that any process for hydrochlorination of myrcene, to be commercially feasible, must result in a favorable yield of the desired geranyl (II) and neryl (III) monochlorides and minimal formation of linalyl (IV) and alpha-terpinyl (VI) monochlorides. It was previously appreciated that the relative proportions of monochlorides (II), (III) and (IV) in the hydrochlorination product reaction mixture could be controlled to some degree by selection of the reaction temperature, gas flowrate and catalyst concentration.
We have now found that when the prior art hydrohalogenation of myrcene (U.S. Pat. No. 3,016,408 to Webb and U.S. Pat. No. 2,871,271 to Booth), is carried out in the presence of a of catalytic amount of a certain kind of organic quaternary salt, then the isomerization of the linalyl chloride product during the hydrohalogenation reaction is shifted to favor formation of the less-substituted allylic chloride, being geranyl chloride (II) and neryl chloride (III).
While it is known in the prior art to employ a copper catalyst in combination with an organic quaternary salt (U.S. Pat. Nos. 3,819,730 to Nakata and 3,836,592 to Gordon) to isomerize an allylic halide to its allylic isomer in a separate reaction step, apart from the formation of the allylic halide by hydrohalogenation of a diene, the shortcomings of the art are evident when applied to the isomerization of linalyl chloride, in particular the use of elevated temperatures which product low yields because of extensive rearrangement to terpinyl chloride, an indesirable side reaction unique to the myrcene hydrochlorides.
The advantages associated with the improved process of the invention are improved overall yield of the more desirable neryl and geranyl chlorides and a greater selectivity of the more important geranyl isomer. Furthermore, because isomerization occurs during the hydrochlorination operation, there is no need for a subsequent, separate isomerization operation.